1. Field of the Invention
This invention relates to optical systems for determining surface irregularities of a workpiece, such as a silicon wafer, having a nominally plane reflective surface.
2. Description of the Prior Art
For certain purposes, it is essential that a workpiece have a substantially flat surface. For instance, in photolithography, such as is used in the manufacture of solid state electronic devices from a silicon wafer, the maximum surface irregularities, such as warpage, that can be tolerated is about 10 micrometers. Various types of optical systems have been developed for determining (e.g., measuring) the surface irregularities of a workpiece having a nominally plane (flat) reflective surface.
One such system employs an interferometer capable of detecting surface irregularities with a precision in height in the Angstrom region. Although this high sensitivity is desirable in the production of optically-flat surfaces, the dynamic range of an interferometer working with visible light is limited to a few micrometers. For such applications as the determination of surface irregularities for a silicon wafer to be used in the manufacture of solid state electronic devices, the sensitivity of an interferometer is higher than is needed and the dynamic range of an interferometer is smaller than is needed.
Non-contact profilers, using scanning techniques and microscopes for generating a height map, have been developed that exhibit both high sensitivity and a large dynamic height range. However, these non-contact profilers are very expensive, costing around $100,000.00.
Other systems have been developed, employing optical imaging, which exhibit relatively large dynamic range, but relatively low sensitivity. One such system, which employs a projection lens to form a defocused image of the surface of a workpiece, such as a wafer, exhibits a sensitivity of only about 15 micrometers (corresponding to a radius of curvature of only one hundred meters). Another such system, in which an image of a known grid pattern reflected from the surface of a workpiece is photographed and the surface irregularities are determined by geometrical distortions of the photographed grid pattern, exhibits a somewhat higher sensitivity of 5-10 micrometers (corresponding to a radius of curvature of 270 meters).
The optoelectronic system of the present invention for determining surface irregularities provides at a relatively low cost a large dynamic range and a high sensitivity. More specifically, the optoelectronic system of the present invention exhibits a sensitivity corresponding to a radius of curvature of 1,000 meters and a dynamic range extending from a radius of curvature of 1,000 meters to a radius of curvature of only 10 meters.